Oversized Piping IPC

[jar546]

In the IPC, there are stipulations regarding 'Water Pipe Sizing' and 'Fixture Units.' With the increasing popularity of low-flow fixtures and sustainable plumbing solutions, how do you handle the pipe sizing given that the traditional fixture unit values might not necessarily represent the reduced flow rates?

 

[steveray]

Also DWV pitch should probably be revisited....With less water in the pipes....

 

[wwhitney]

In the IPC, there are stipulations regarding 'Water Pipe Sizing' and 'Fixture Units.'

To my understanding, "water supply fixture units" are not required to be used in designing water distribution systems, unless your jurisdiction has formally adopted IPC Appendix E as mandatory. WSFUs are simply one way to choose a design demand flow rate for a collection of fixtures. To elaborate:

Section 604 of the IPC covers the "design of building water distribution" and provides minimum pipe sizes for the connection to a single fixture (often just 3/8"), as well as minimum flow rates to be provided, along with minimum residual pressures. But the latter are design criteria and Section 604 doesn't specify a design methodology to achieve those criteria.

Appendix E is on the "sizing of water piping system," but it provides the caveat that "the provisions contained in this appendix are not mandatory unless specifically referenced in the adopting ordinance." This section provides two design methods, the first computational, and the second tabular. These methods use WSFUs to choose the design demand flow, but if Appendix E has not been formally adopted as mandatory, a designer could use some other rational method to determine the design demand flow.

Cheers, Wayne

 

[Mech]

When it comes to public lavs, I may swerve away from water fixture supply units, depending which pipe section I am considering. From the 2018 IPC, Table 604.3 says to size the fixture supply lines for 0.4 gpm. One hot and 1 cold water line * 0.4 gpm each = 0.8 gpm. Four lavs * 0.8 gpm each = 3.2 gpm. Using fixture units, 4 lavs * 2 wfu = 8 wfu, which translates to 12.8 gpm which is four times the rate used for sizing supply lines. If the pipe is serving a bunch of flush valve fixtures, I will just add 2 wfu per lav, and it doesn't change the total flow rate that much.

Similarly, shower heads are limited to 2.5 gpm at 80 psi per Table 604.4. Four showers at 2.5 gpm = 10 gpm. Using water fixture units, four showers at 4 wfu = 16 wfu which is 18 gpm. Again, depending which pipe section I am considering, I will use the max shower head flow rate (or individual supply tube flow rates of 2.5 gpm) * number of showers instead of wfus to size the pipe.